The present invention relates to a mixer for use in the development housing of a photocopying machine, serving to lift, and mix the developer mixture, and to level the concentration of the mixture with a single element.
It is well known in the prior art that electrophotographic copying machines employ a two component brush developer to produce an image on an imaging surface by selectively developing toner particles from a mixture of carrier and toner. The quality of the resulting image depends upon the toner charge (tribo) and the toner concentration which is brought into contact with the imaging surface from the development unit.
Triboelectric charging of the toner is effected by mixing or tumbling the toner and the carrier within the development unit. Indeed, it is necessary to stir, mix, lift, and drop the toner/carrier mixture in order to create the triboelectric charge required for the development process. It is well known in the prior art to provide such a means in a development unit.
One consequence of image development onto the imaging surface is the removal of toner from the developer housing within the development unit thereby reducing the concentration of toner within the carrier. In order to maintain the toner concentration required for good and consistent image quality from the photocopying process, it is necessary to add toner to the development housing to replace that which is removed by the imaging surface. Toner, which is typically in powder form, is usually added uniformly along the entire length of the development unit--the length of the development unit usually being approximately equal to the maximum width of the copy media used by the photocopy apparatus. However, due to the spatial variations of the photocopied image (for example, most common applications may have more dark portions toward the center than around the edges of the image), toner is selectively removed at different rates across the width of the development housing. This selective removal of toner combined with the uniform addition of toner presents a problem in that the local toner concentration varies along the length of the developer system. This problem is particularly severe in long developer housings such as those used by engineering or architectural drawing copy machines.
The prior art has attempted to remedy this deficiency by various means such as the use of twin oppositely feeding augers, a paddle wheel in conjunction with one or more augers, and hollow paddle wheels containing screw shaped baffle systems. These prior art devices are deficient in that they are unnecessarily complicated, and therefore are failure prone. Furthermore, these devices may reduce the developer usable lifespan and may still not result in uniform or complete mixing over the full length of the development system.
It is therefore an object of this invention to provide an apparatus which will uniformly mix the developer mixture in a development system in a photocopier notwithstanding the non-uniform depletion of the toner in the developer system.
It is a further object of this invention to provide an apparatus which will agitate the developer mixture in a photocopier so as to triboelectrically charge the mixture.
It is a still further object of this invention to provide an apparatus which will not reduce the usable life of the developer mixture.
A still further object of this invention to provide the aforementioned objects with a simple, single unit.